1. Field of the Disclosure
The present disclosure relates to a capacitance detection apparatus such as a capacitive sensor that detects capacitance and relates to an input apparatus such as a touchpad including the capacitance detection apparatus.
2. Description of the Related Art
An apparatus, such as a touchpad or touch panel, that includes a sensor for detecting a contact position of an object (for example, a finger, a pen) has been widely used as an input interface of an information apparatus such as a notebook PC, a tablet terminal, a smart phone, and the like. Existing various types of sensor that detects a contact position of an object include, for example, a resistance film method and a capacitance method. Recently, a sensor based on a capacitance method, which supports a “multi-touch” capability enabling detection of a plurality of contact positions, has been increasingly employed.
A sensor based on the capacitance method detects a voltage converted from a minute change in capacitance and, hence, is likely to be influenced by external noise. In the capacitance detection circuit disclosed in US2014/0035601A1, a charge provided from a sensor is continuously integrated by switching between the directions in which a capacitor inserted in the feedback path of a charge amplifier is connected during the rising period and falling period of the drive signal from the sensor. This allows external noise to be averaged and attenuated and, hence, influence from external noise is unlikely to be generated.
FIG. 6 is a schematic diagram illustrating the configuration of the capacitance detection circuit disclosed in FIG. 13 of US2014/0035601A1. The existing circuit illustrated in FIG. 6 operates as a delta-sigma-type analog-digital converter including a comparator 102 with one-bit output and a digital filter 103.
In FIG. 6, a capacitor C1, a switch circuit 100, and an operational amplifier 101 form a charge amplifier accumulating a charge Qin transferred to an input terminal Tin. This charge amplifier continuously accumulates (integrates) positive and negative charges Qin using the capacitor C1 by switching the connection direction of the capacitor C1 inserted into a feedback path in accordance with the polarity of the charge Qin. Hence, the amplitude of an output voltage Vamp of the operational amplifier 101 increases with time. As illustrated in the timing chart of FIG. 7, when the positive charge Qin and the negative charge Qin are each accumulated once, a signal Lb enters a valid state and the comparator 102 operates, whereby the output voltage Vamp of the operational amplifier 101 is compared with a reference voltage. Hence, a signal CMP, which is the comparison result of the digital filter 103, is not output before the positive and negative charges Qin are each read once. When the period of the output of the signal CMP increases, a time required before the detected capacitance of a predetermined bit length is obtained increases.
The size of an input surface of the sensor of a recent touchpad or the like is has been tending to increase. As a result, the number of detection positions on the input surface is increasing and, hence, the number of detection signals obtained from the whole input surface is also increasing. On the other hand, in order to accurately capture the movement of a finger during a touch operation, a sensing speed needs to be a predetermined speed or higher and, hence, a time (scanning cycle time) for acquiring detection signals from the whole input surface is not allowed to increase so much even when the number of detection signals increases. Consequently, a sensing time at each detection position needs to be decreased, i.e., the speed of detecting capacitance needs to be increased.
Further, in a state in which a finger has been lifted from the input surface, or the finger is gloved, for example, a minute change in capacitance needs to be detected with high accuracy. In the case of a low detection speed, there is a problem of an increased influence of noise because noise immunity is reduced due to averaging.
In view of these situations, it is an object of the present invention to provide a capacitance detection apparatus and an input apparatus enabling an increase in the speed of detecting capacitance.